Abstract

Background

Interleukin-1beta (IL-1β) is a pro-inflammatory cytokine that can be produced in the
central nervous system during inflammatory conditions. We have previously shown that
IL-1β expression is altered in the rat brain during a morphine tolerant state, indicating
that this cytokine may serve as a convergent point between the immune challenge and
opiate mediated biological pathways. We hypothesized that IL-1β up-regulates opioid
receptors in human astrocytes in both untreated and morphine-desensitized states.

Methods

To test this hypothesis, we compared the basal expression of the mu (MOR), delta (DOR),
and kappa (KOR) opioid receptors in the human U87 MG astrocytic cell line to SH-SY5Y
neuronal and HL-60 immune cells using absolute quantitative real time RT-PCR (AQ-rt-RT-PCR).
To demonstrate that IL-1β induced up-regulation of the MOR, DOR and KOR, U87 MG cells
(2 x 105 cells/well) were treated with IL-1β (20 ng/mL or 40 ng/mL), followed by co-treatment
with interleukin-1 receptor antagonist protein (IL-1RAP) (400 ng/mL or 400 ng/mL).
The above experiment was repeated in the cells desensitized with morphine, where U87
MG cells were pre-treated with 100 nM morphine. The functionality of the MOR in U87
MG cells was then demonstrated using morphine inhibition of forksolin-induced intracellular
cAMP, as determined by radioimmunoassay.

Results

U87 MG cells treated with IL-1β for 12 h showed a significant up-regulation of MOR
and KOR. DOR expression was also elevated, although not significantly. Treatment with
IL-1β also showed a significant up-regulation of the MOR in U87 MG cells desensitized
with morphine. Co-treatment with IL-1β and interleukin-1 receptor antagonist protein
(IL-1RAP) resulted in a significant decrease in IL-1β-mediated MOR up-regulation.

Conclusion

Our results indicate that the pro-inflammatory cytokine, IL-1β, affects opiate-dependent
pathways by up-regulating the expression of the MOR in both untreated and morphine-desensitized
U87 MG.